1. Field of the Invention
The present invention relates to a lens barrel and an imaging apparatus.
2. Description of the Related Art
There has been proposed a lens barrel including a zoom mechanism that moves a plurality of optical lenses in the optical axis direction so as to change an imaging magnification. In the lens barrel, a lens holding frame for holding a lens is engaged with a cam groove of a cam barrel for rotation and the rotation of the lens holding frame is restricted by a linear barrel so as to move in the optical axis of the lens barrel. In addition, the lens holding frame of a focus group is independent of the lens holding frame of another group engaged with the cam barrel and advances or retracts by a screw attached to a focus motor, a nut and a rack which mesh with the screw in the optical axis direction according to the rotation of the screw. The screw-type focus mechanism is widely used. Japanese Patent Laid-Open No. 2000-193877 discloses a device that corrects the amount of movement of the lens holding frame in the optical axis direction by detecting the rotation of the screw using a signal detecting unit such as a photo interrupter, a photo reflector, or the like.
FIG. 9 is a diagram illustrating an example of a configuration of a device disclosed in Japanese Patent Laid-Open No. 2000-193877. As shown in FIG. 9, a stepping motor for focus drive is provided with a pulse plate 121 and a photo interrupter 122 for detecting a signal of the pulse plate 121 at its distal end via a screw 109. When an electric current is passed through the stepping motor, the screw 109 rotates, a nut 118 which meshes with the screw 109 advances or retracts in the optical axis direction, and a focus lens holding frame 108 engaged with the nut 118 and a lens 107 also advance and retract following the movement of the nut 118. At this time, the pulse plate 121 which is integrally attached to the screw 109 also rotates, and the signal of the pulse plate 121 is counted by the photo interrupter 122. When a difference occurs between a feed pulse and a count pulse in the camera operating temperature, the difference can be corrected so as to obtain appropriate focus of the image.
However, in the device disclosed in Japanese Patent Laid-Open No. 2000-193877 as shown in FIG. 9, a lens holding frame cannot be entered into a portion L at which the pulse plate 121 and the photo interrupter 122 are attached to a base plate 101 in the range of use of space in the focus mechanism. Thus, in the device disclosed in Japanese Patent Laid-Open No. 2000-193877, a space other than the portion is used as a focus stroke. Thus, a distance which can be used as a focus stroke within the space occupied by the focus mechanism in the optical axis direction is shortened, resulting in degradation of the specification of focus. In addition, since the pulse plate 121 is attached to the distal end of the screw 109, stability of counting of fine signals may be inhibited by the large influence of tilt or vibration of the long screw 109.